The present invention is directed to a print head and, in particular, to one that is designed to meet the requirements for microarray printing.
Microarrays have requirements that differ from most image printing applications, and therefore conventional print head designs are not optimal for this application. Microarrays contain a matrix of sites onto which liquid reagents are deposited. A sample applied to the microarray reacts at different sites containing different reagents. The results of the reactions are usually interrogated optically, providing a highly multiplexed analysis of the sample. Requirements of typical microarrays are:                Many different reagents to be deposited (typically 20-10000)        Few replicates (sites containing a common reagent) (typically 2-10)        Replicates should be distributed across the area of the array (in widely separated rows and columns, typically at least 1 mm separation)        Total number of spots is typically 100-1,000,000        Spot spacing is typically 30-1000 microns        Array size is typically 3 mm×3 mm to 75 mm×75 mm        Spots must be satellite-free and clearly separated from neighbours to enable automated analysis of the microarray        Reagents can include DNA, proteins, antibodies, cells and cell fragments and other materials including suspensions        Reagents may have stringent material compatibility requirements        Thorough and automated cleaning and reservoir refill is required.        
For high speed and reliability, non-contact ink-jet printing is beneficial for microarray manufacture. Contact pin methods are relatively slow and have high pin maintenance requirements. Additionally, fixed print heads are preferred to avoid mechanical complexity associated with a scanning head.
Conventional industrial ink-jet heads (e.g. Dimatix, Xaar) can be used to manufacture microarrays. These typically have large numbers of nozzles (128 or more) on a narrow pitch (254 microns) sharing a common reservoir. The number of print heads required is equal to the number of reagents, increasing the size and cost of manufacturing equipment and requirements for print head registration. Replicates are usually widely spaced so the narrow nozzle pitch is not required and adds complexity.
Single nozzle print heads (e.g. piezo tube type) are an alternative method for fabrication of microarrays. These can be arranged at the required separation to match replicate locations but require registration of (no. of replicates×no. of reagents). This would be very challenging.
Both methods above suffer from poor cleanability and low tolerance to air bubbles, due to the presence of narrow channels and the compression-chamber ejection mechanism.
EP0615470 describes a circularly-actuated piezoelectric-driven nozzle, which has more robust construction and is more capable of ejecting viscous liquids than linear bending mode devices such as those described in EP1071559. However a multiple nozzle print head based on the circularly-actuated device described in EP0615470 would suffer from high levels of crosstalk. The mounting structure in the invention described below circumvents this problem.